It is known to withdraw solids, such as catalyst particles which gradually have become deactivated, e.g. during operation of hydrocarbon processes in which they are employed, from vessels, especially reactors which are equipped with a movable catalyst bed. Such vessels usually comprise one or more screen sections for separating the reactor effluent from the catalyst, the vessel being provided with separate outlet means for effluent and catalyst in a lower part of the vessel. Fresh and/or regenerated catalyst can be fed continuously or at intervals at the top and spent catalyst is removed from the bottom of the vessel via an outlet system containing a rotating valve as for instance as described in U.S. Pat. No. 4,590,045 issued May 20, 1986.
Since catalyst fines and smaller (broken) catalyst particles have a tendency to accumulate near the reactor bottom they will block the rotating valve and thus reduce the withdrawal flow of spent catalyst.
It has now been found that the problem encountered with the use of rotating valve systems can be solved in a different manner.
It should be noted that it is known from Reed & Mason, Journal of Pipelines (5, 1985, 67-75) to unload ships in an economically attractive way with an air-driven withdrawing system comprising a suction nozzle which consists of two tubes mounted co-axially, wherein the inner conveying tube has to protrude from the outer tube in order to obtain meaningful results.